1. Field of the Invention
The present invention relates to a grinding machine wherein in response to a signal issued from a sizing device when a workpiece being ground reaches a predetermined size, the infeed movement of a grinding wheel carrier is halted prior to a fine grinding operation for removing the flexing of the workpiece.
2. Discussion of the Prior Art
Up to now, there is known a numerical control grinding machine wherein the grinding mode is changed in response to a first sizing signal issued when a workpiece is ground to a predetermined size. However, the workpiece is ground by a grinding wheel at a predetermined infeed rate so that the workpiece is flexed toward the feed direction of the grinding wheel. Where the infeed movement of the grinding wheel is halted upon detecting the first sizing signal, a problem arises in that the workpiece is continued to be further ground an amount due to its spring-back motion which is caused by the flexed workpiece. Hereafter, the grinding caused by the spring-back motion will be referred to as "drifty grinding", and similarly, the grinding amount in such "drifty grinding" will be referred to as "drifty grinding amount". Therefore, it is necessary to add the drifty grinding amount to a set size which determines the time point to issue a second sizing signal for the initiation of a fine grinding operation.
By the way, in the case of CBN (Cubic Boron Nitride), because the grinding capability is small right after each truing operation as well as at the first use of new grinding wheel, the drifty grinding amount increases. Further, as the number of the ground workpieces increases, the drifty grinding amount decreases by reason that the cutting edge of each abrasive grain is broken so that the grinding capability becomes large. Therefore, if the set size of the workpiece diameter upon issuance of the first sizing signal is constant without regard to the number of ground workpieces, the grinding accuracy may be degraded with increase in the number of workpieces, and in addition, the grinding cycle time and the feed amount of a rest jaw may be varied.
In order to solve such problems, in the prior art, the set size of the workpiece diameter upon which the first sizing signal is issued is diminished with increase in number of ground workpieces based on the experimental relation between the number of ground workpieces and the cutting quality of the CBN grinding wheel. As a consequence, the drifty grinding amount is controlled to be nearly constant throughout all the grindings of workpieces.
However, the relation between the number of ground workpieces and the cutting quality varies with the kind of CBN grinding wheel used, the material of workpieces, the grinding amount par workpiece and so forth. For this reason, in the prior art method wherein the set size which determines the time point to issue the first sizing signal is altered a predetermined value each time a workpiece is ground, there is raised another problem that the relation between the set size above and the number of ground workpieces has to be altered to meet respective conditions such as the kind of grinding wheels, the material of workpieces, the grinding amount par workpiece and so forth.